The present invention relates to a class-D amplifier which generates a PWM-modulated pulse based on a bit stream of one bit acquired through ΔΣ modulation, to thus drive a load.
Such a class-D amplifier has been proposed which applies a ΔΣ-modulated bit stream of one bit to a PWM (Pulse Width Modulation) modulator as an input signal showing original sound and which drives a load, such as a speaker, by the PM-modulated pulse output from the PWM modulator (refer to; for example, JP-A-2004-128750), thereby reproducing an audio signal. The class-D amplifier of this type has a simplified circuit configuration and has an advantage of being easily embodied as an integrated circuit.
Incidentally, during ΔΣ modulation, quantization noise is shifted toward a high-frequency domain due to so-called noise shaping at the time of generation of a bit stream. Therefore, the bit stream acquired through ΔΣ modulation includes a large amount of quantization noise at high frequencies. Such quantization noise at high frequencies is lessened to a certain extent by integration processing performed in the PWM modulator after the bit stream has been input to the PWM modulator. However, the amount of attenuation is not sufficient. Therefore, the quantization noise insufficiently attenuated by PWM modulation processing of the PWM modulator induces aliasing noise, and a frequency band of the aliasing noise extends to an audio band occupied by spectra of original sound, thereby posing a problem of deterioration of quality of sound reproduced by the class-D amplifier. in order to avoid occurrence of such a problem, a filter for eliminating noise near a PWM modulation frequency from the bit stream must be provided in a preceding stage of the PWM modulator. However, such a filter is embodied as a large-scale filter, which raises a problem of a class-D amplifier becoming expensive.